JPH07196831A - Polytetrafluoroethylene porous membrane and method for producing the same - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a porous membrane composed of a fibrous structure in which a PTFE molding is stretched in an unsintered state and substantially free of large-sized knots, and a method for producing the same. [Structure] A polytetrafluoroethylene porous film formed by stretching a non-sintered polytetrafluoroethylene in an unsintered state, the surface morphology of which is observed by a scanning electron microscope. A polytetrafluoroethylene porous membrane comprising a fibrous structure substantially free of large knots. 100% / sec or less, preferably 50% / sec or less of the polytetrafluoroethylene unsintered body at a temperature of less than 327 ° C,
More preferably, the method for producing a polytetrafluoroethylene porous membrane is characterized by stretching at a stretching area ratio of at least 50 times in the direction of uniaxial or more at a stretching speed of 20% / sec or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polytetrafluoroethylene (hereinafter referred to as PTFE) porous membrane, a fired body thereof and a method for producing them. The PTFE porous membrane according to the present invention is used as an excellent air filter material for clean rooms in the semiconductor industry, pharmaceutical industry and the like.

[0002]

2. Description of the Related Art A technique for extruding an extruded body (sometimes rolled and formed) made of a mixture of PTFE fine powder and a lubricant, that is, a so-called paste molded body, by removing the lubricant and then stretching it to make it porous. Various things are known. When this technology is classified according to the heat treatment state during stretching, (1) a method of stretching a molded body as an unsintered body (for example, Japanese Patent Publication No. 42-13560, Japanese Patent Publication No. 51-
18991, JP-B-56-45773, JP-B-56-1
7216), and (2) a method of stretching as a fired body (for example, JP-A-53-55378 and JP-A-55-5537).
9, JP-A-59-109534, JP-A-61-2074
46) and (3) a method of stretching as a semi-baked body (for example, JP-A-58-145735, JP-A-59-1528).
No. 25, JP-A-3-221541).
In either case, the stretching is performed at the melting point of PTFE (327
The heating temperature is less than (.degree. C.), but a part may be stretched while being heat-treated.

Among these techniques, the method (1) of stretching in the form of a green body is a pioneering technique in this technical field. This method basically evaporates and removes (drys) the liquid lubricant from the PTFE paste molded body, and then stretches the unsintered body at a heating temperature lower than the melting point of PTFE at a relatively high speed in a uniaxial or more direction. It is a thing. The PTFE porous body produced by this method has a fiber structure including large size knots larger than 1 μm interconnected by extremely small fibers, and has a high porosity of 40 to 97% and an extremely high strength. Is. The characteristic of this method is that the thickness of the molded body is hardly reduced when the draw ratio is increased. This means that even if the draw ratio is increased,
This means that the number and size of the spaces created by the nodules and fibers are increased. That is, it means that as the draw ratio increases, the pore diameter increases, the space expands, and the porosity only increases.

The method (2) of stretching in the form of a fired body is a method of stretching the molded body after heating and firing it to a temperature above the melting point of PTFE, or while heating and firing above the melting point. However, these methods can only draw in a direction perpendicular to the rolling direction, and it is generally difficult to produce a porous body having a high porosity.

The method of stretching in the form of the semi-baked body of (3) is
The molded body is heat-treated before stretching, and is disclosed in JP-A-59-152825.
2 after being brought to a certain "semi-baked state" as defined in
This is a method of stretching in the direction of the axis or more. The feature of this method is that the pore diameter becomes smaller as the draw ratio becomes larger, and an extremely thin membrane having a structure consisting of fibers having substantially no large size knots is obtained. For example, according to Japanese Patent Laid-Open No. 5-202217, the PTFE porous membrane produced by this method is a porous membrane having a structure consisting only of fibers having substantially no knots and having an average pore size of Usually 0.5 to 0.2 μm, which is extremely small, and the film thickness is 1/20 to 1 of the thickness of the molded product before stretching.
The feature is that it is reduced to about / 100. However, this method has a very narrow temperature range for obtaining a "semi-baked state" which is convenient for the subsequent stretching, and it is extremely difficult to realize such a state in practice.

As described above, in the method of stretching a PTFE molded body as an unsintered body, it has a fiber structure composed of fibers and knots interconnected by the fibers, has a relatively large pore diameter, and a film thickness. While a large porous film can be obtained,
On the other hand, it has been considered so far that the method of stretching while maintaining the semi-baked state can obtain an extremely thin film having a small pore size having a structure consisting of substantially large-sized fibers without knots. In other words, an extremely thin membrane having a small pore size, which has a structure composed of only fibers having substantially no large-sized knots, has been hitherto semi-baked as described in JP-A-5-202217. It has been said that it cannot be produced by a method other than the method of stretching in the state. For example, in the method of stretching in an unfired state, it has been considered necessary to reduce the stretching ratio in order to reduce the pore size.

As a prior art similar to the present invention, JP-B-56-45773 relating to a method of producing a rod or a film by stretching at a high ratio of 50 times or more of the original length by a method of stretching in an unsintered state. There is an invention of the issue. But,
An object of the present invention is to produce a high-strength and highly porous material, and the porous material in that case means a structure having fibers and knots. Then, in order to obtain such a porous material, it is an essential requirement to stretch at a stretching rate of at least 2000% / sec. On the other hand, the present invention aims to achieve the formation of a porous film having a structure consisting of substantially no-dimensional fibers without knots by stretching in a non-fired state, and as a means therefor. It is an essential requirement that the stretching is performed at a stretching rate of 100% / sec or less, preferably at a stretching rate of 50% / sec or less, and more preferably at a stretching rate of 20% / sec or more.

[0008] Here, an overview of the process for producing a porous product by subjecting PTFE fine powder to paste molding and then stretching in an unfired state will be made. PTF suitable for this method
E fine powder has a low content of amorphous, 9
PTFE having a crystallinity of 8% or more is preferred. When such a PTFE fine powder is mixed with a lubricant such as mineral spirit or naphtha, it absorbs this and becomes a paste. It is well known that this PTFE paste can be economically molded by a molding method such as calender molding which gives shear deformation in addition to extrusion molding. Pastes are typically formed into various cross sectional shapes such as tubes, rods and tapes. After molding, the lubricant is usually removed from the molded body by drying. Next, the molded body from which the lubricant has been removed is made into a porous structure by stretching, and it has already been described that there are three types of steps from this molding to stretching. In the method of stretching in the unfired state, the molded body from which the lubricant has been removed has a temperature below the melting point of PTFE,
Preferably, it is stretched in a uniaxial or more direction at a high temperature below the melting point and close to the melting point. The molded product having a porous structure after stretching is usually subjected to a so-called baking treatment, in which the molded product is heated to a temperature equal to or higher than the melting point of PTFE and then cooled in order to fix the fine structure of the molded product. The degree of baking of the product is controlled by adjusting the height of the maximum temperature and the length of holding time at that temperature depending on the application of the product. In addition, depending on the application, there are cases where the baking treatment is not performed at all.

The shaped body is made porous by the stretching treatment in the unsintered state, and its fine structure depends on the stretching temperature, the stretching speed,
It is affected by the draw ratio. This structure consists of a large number of fine fibers and knots connected to each other by the fibers, and the size and arrangement of the knots change depending on the stretching conditions. For example, when stretched in the uniaxial direction, the knot portions are arranged in an island shape at right angles to the stretch direction, and the fibers that connect the knot portions to each other are oriented parallel to the stretch direction. When stretched in the biaxial direction, the nodule is composed of particles or an aggregate of several to several hundred fine powder particles, and the fibers connecting them are oriented in the two-dimensional direction from the nodule. The degree depends on the stretching conditions. In the conventional porous membrane, the fiber diameter is about 0.1μ
The size of the nodule is as large as about m, and the size of the nodule is as large as about 400 μm in some cases. An electron micrograph of a typical example of the fiber structure formed by the uniaxial stretching is shown in FIG.

A feature of the porosification by stretching in the unfired state is that the dimension of the molded body in the direction perpendicular to the stretching direction (vertical direction) does not change in the stretching step. In other words, the thickness and width of the molded body change in uniaxial stretching, whereas the thickness of the molded body changes only slightly in biaxial stretching. This indicates that the increase in volume is due to an increase in porosity, that is, a decrease in density. This increase in porosity is caused by an increase in voids, that is, spaces between the nodules, and also by an increase in the number and length of fine fibers and an increase in the size of the spaces. Therefore, it is essentially difficult to produce a film thinner than the original thickness of the molded product before stretching by the method of stretching in an unfired state.

[0011]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
It is an object of the present invention to provide a porous membrane composed of a fiber structure in which a molded body is stretched in an unfired state and substantially free of large-sized knots, and a method for producing the porous membrane.

[0012]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that when a PTFE molded article is stretched in an “unbaked state” to form a stretched film, It was found that, when the stretching speed is extremely slow and the stretching area ratio is 50 times or more, a porous membrane having a fiber structure can be obtained, which is surprisingly free of large-sized knots. Has been completed.

That is, according to the present invention, there is provided a polytetrafluoroethylene porous film formed by holding a non-sintered polytetrafluoroethylene body in an unsintered state and stretching it, and in a surface morphology observed by a scanning electron microscope. There is provided a polytetrafluoroethylene porous membrane, which is characterized by comprising a fibrous structure substantially free of knots having an average size larger than a circle having a diameter of 1 μm.

Further, according to the present invention, the unbaked polytetrafluoroethylene is treated at a temperature of less than 327 ° C.
Polytetrafluoro characterized by being stretched at a stretching area ratio of at least 50 times in the direction of uniaxial or more at a stretching speed of 0% / sec or less, preferably 50% / sec or less, more preferably 20% / sec or less. A method for producing an ethylene porous membrane is provided.

Further, there is provided a polytetrafluoroethylene porous film fired body obtained by heating the porous film to a temperature of 327 ° C. or higher.

Further, there is provided a method for producing a polytetrafluoroethylene porous film fired body, which comprises heat-treating at a temperature of 327 ° C. or higher after obtaining the tetrafluoroethylene porous film by the above method.

The present invention will be described in detail below, including the porous membrane and the method for producing the same. In the method for producing a porous film of the present invention, a high crystallinity PTFE fine powder with a small amount of amorphous is used as a raw material, which is formed into a paste with a lubricant and then formed by a shearing force, The same steps as the conventionally known steps can be adopted as the steps up to the removal of the lubricant from the body. To produce a PTFE molded product, as a raw material thereof, a PTFE fine powder having a low amorphous content and a crystallinity of 98% or more is used. A lubricant such as mineral spirit or naphtha is uniformly mixed with this PTFE fine powder to form a paste. Next, this paste is molded into a form according to the purpose of the product by a molding method such as extrusion molding or calender molding that imparts shear deformation. Usually, it is formed into a tape shape, but the shape is not necessarily limited to this, and may be formed into various sectional shapes such as a rod and a tube depending on the purpose of the product. After molding, the lubricant is removed from the molded body by drying. For drying, in addition to the method of heating the lubricant above the boiling point to remove the lubricant,
This may be accompanied by a gas flow such as air or nitrogen, or optionally dried under reduced pressure. Also, a method of extracting and removing with an extractant having compatibility with a lubricant and having a boiling point lower than the boiling point of the lubricant and then drying can be employed. Then, the molded body from which the lubricant has been removed
Temperature below 327 ° C (melting point of PTFE), preferably 2
It is stretched in a direction of uniaxial or more in a temperature range of 00 ° C to 320 ° C. In this stretching, a stretching rate of 100% / sec or less, preferably 50% / sec or less, and more preferably a relatively slow stretching rate of 20% / sec or less is adopted. The stretching at this time is performed so as to achieve an area ratio of 50 times or more the area of the original molded body. Stretch 2
The molded body may be re-gripped into a pin frame of a stretching machine in multiple stages such as three stages, in which case the first stage is stretched at a conventional high stretching speed, and then the second stage is performed according to the present invention. Stretching may be performed at a relatively slow stretching speed. In short, it is necessary to employ a relatively slow stretching speed according to the invention in the final stage of stretching. The number of stages of stretching method to be adopted is determined in consideration of economic efficiency. By this stretching, a porous membrane having a peculiar membrane structure composed of a fiber structure having substantially no large-sized knots is formed. This porous membrane may be optionally subjected to a firing treatment of heating to a temperature of 327 ° C. or higher and then cooling in order to fix its fine structure, but this is not always necessary, and sometimes the firing treatment is not performed at all. You don't have to.

The term "stretching speed" as used in the specification refers to the ratio (percentage / second) of the separating speed of the frame to the initial inter-frame distance before stretching when the opposite pin frames are stretched. In the case of stretching between a pair of opposing rolls having different speeds, it is defined as the ratio (percent / second) of the rotational speed difference of the rolls to the distance between the rolls.

The term "stretch ratio" as used in the specification means the ratio (times) of the final inter-frame distance after stretching to the initial inter-frame distance before stretching when stretching is performed by separating the opposing pin frames. , Or defined as the ratio (percentage) of the distance the molded body is stretched (the value obtained by subtracting the initial interframe distance before stretching from the final interframe distance after stretching) with respect to the initial interframe distance before stretching. In the case of stretching between a pair of rolls having different speeds, the ratio of the rotation speeds of the pair of rolls (times), or the stretched distance (one pair of rolls) of the molded body with respect to the rotation speed of the first roll. It is defined as the ratio (percentage) of the difference in the rotational speed of the rolls. Therefore, for example, a draw ratio of 5 times is 400% when expressed as a percentage.

The PTFE porous film of the present invention has a diameter of 1 μm in a surface morphology observed by a scanning electron microscope (SEM).
The fibrous structure is substantially free of knots having an average size larger than a circle of m ”. As described above, the porous film obtained by stretching in the unsintered state by the conventional method is characterized by the presence of a large number of large-sized knots, but in the porous film of the present invention, such a conventional porous film is The large size nodules seen are virtually absent.
In the porous membrane of the present invention, there are large-diameter portions larger than the fiber diameter, such as the bent portion of the fiber, the branched portion of the fiber, or the central portion where the fiber extends radially. However, such a large-diameter portion is substantially different from the large-sized knotted portion found in the conventional porous body, and rather should be excluded from the conventionally-known knotted portion. Although conceivable, since it is difficult to clearly distinguish it from a conventional knot, in the present specification, it is defined as a knot for convenience.

In the porous membrane of the present invention, the size of the nodule is
The average size is 1 μm or less, which is smaller than a circle having a diameter of 1 μm. In this case, the size of the nodule is determined by measuring the SEM photograph. In determining the size of the nodules, arbitrarily or non-conception to selected about 1200μm ² ~500μm ² ranging from the SEM field,
Preferably, the size must be statistically determined based on an SEM photograph in which an area of 880 μm ² fills one visual field. Also, the size should not be determined by taking a very exceptional field of view, but by a normal and normal sensation based on the average field of view. The measurement of the size of the nodule on the SEM photograph may be performed by a mechanical / electronic method using an image analysis device, or by a hand.
Generally measured by hand. Also, "average 1 μm or less"
Means an average number in a statistical sense and should not be emphasized with exceptional numbers. As described above, the average size of the nodules is 1 μm or less. However, when actually evaluating the nodules, it may be difficult to determine where the boundaries between the nodules and the fiber parts are. The nodule is elongated and 1 μm in the minor axis direction.
Although it is difficult to judge whether or not the nodule is larger than the circle because it enters the circle but protrudes in the long axis direction, there are rare cases where it is difficult to make the judgment in reality. In such cases, judgment should be made based on the following criteria. That is, there is a portion in which a circle with a diameter of 1 μm is included in any part of the nodule (including a fiber part where the boundary with the nodule is unclear in some cases), and the nodule has a “diameter”. It is larger than a circle of 1 μm ”.

When the PTFE porous membrane of the present invention is specified, it is not appropriate to evaluate it by the fiber: nodule area ratio or the occupied area ratio of the nodule. This is because it is difficult to exclude simple intersections, confluences, and bending points of fibers from the knots. Further, although the porous membrane of the present invention is extremely thin, it still has a three-dimensional structure, and the membrane thickness becomes smaller as the draw ratio becomes larger, and as a result, the membrane becomes fiber-like in the SEM field of view. This is because the number of knots changes as the number of overlapping fibers increases. In the nodules present in the fibrous structure constituting the porous membrane of the present invention, the number of large nodules exceeding 1 μm is 0 to 1 per photographing area 880 μm ^{2 as} shown in the examples, It is above zero.

The thickness of the PTFE porous membrane of the present invention is reduced to 1/20 to 1/400 of the thickness of the molded product before stretching, is usually 10 μm or less, and its lower limit is not particularly limited. However, in general, it is 0.1 μm. The average pore size in the porous membrane of the present invention is extremely small,
It is 0.5 μm or less. The lower limit of the average pore size is not particularly limited, but is generally 0.1 μm.

[0024]

The porous PTFE membrane of the present invention is disclosed in
As pointed out in Japanese Patent No. 202217, the conventional method of stretching in an unfired state could not be obtained at all. Therefore, the present invention has a thin PTFE porous structure having a structure consisting only of fibers substantially free of large-sized knots as described above, which has been conventionally considered to be produced only by a drawing method in a semi-fired state. The present invention provides a new method for realizing a membrane and its manufacture.

The PTFE porous membrane of the present invention can be used not only as an air filter but also as a battery diaphragm, a humidifier diaphragm, or a pervaporation diaphragm. In addition, it can be used as a fabric material used in applications requiring a clean environment.

[0026]

The present invention will be described in detail below with reference to examples. The "average pore size" shown in the examples below is for Coulter
Mean flow pore size (MF) measured with a porometer (Coulter Electronics (USA))
P). The "film thickness" is the SM-1201 dial gauge manufactured by Teclock Co., Ltd. (1/1000 mm
The thickness of the porous film is measured using the
It is a value obtained by measuring the total film thickness by stacking two films and dividing the total film thickness by the number of stacked films.

Examples 1 to 5 PTFE fine powder ("Polyflon Fine Powder F104" manufactured by Daikin Industries, Ltd.) manufactured by a usual method of paste extrusion, roll rolling, and lubricant drying, and having a thickness of 0.2 mm and a width of 150 mm. Unbaked tape,
Using a biaxial stretching machine, stretching temperature 300 ° C, stretching speed 50%
First, the film was stretched about 5 times (400%) in the width direction (first stretching), and then simultaneously stretched in the biaxial direction (second stretching) under the condition of / sec. The stretching in the second stretching is the same in each direction of the two axes, and is 2.0 times (100 times in Example 1).
%), 3.0 times (200%) in Example 2, 4.5 times (350%) in Example 3, and 6.0 times (50% in Example 4).
0%), and in Example 5, it was drawn at five draw ratios of 7.5 times (650%). That is, the final stretch area ratio is 5.0 in Example 1, including the first stretch.
× 2.0 × 2.0 = 20 times, 45 times in Example 2, 101 times in Example 3, 180 times in Example 4, and 281 times in Example 5. After stretching, the porous membrane was heated and baked at 370 ° C. for 5 minutes with all four sides fixed.

In each case, the film thickness after stretching was greatly reduced as compared with the original unsintered unstretched tape, and the thickness was drastically reduced when the stretching ratio was increased. SEM the film surface after stretching
When the area expansion ratio was 45 times or less, the presence of large-sized knots was clearly observed, but when it exceeded 45 times, the large-sized knots were scarcely observed, and the structure was changed to fibers. . The film strength improved as the area expansion ratio increased. When the expansion area ratio was 45 times or more, the porosity became almost constant regardless of the expansion ratio.
The Gurley number decreased sharply as the stretched area ratio increased.

Reference Example 1 The porous film after the first stretching used in the second stretching of Examples 1 to 5 is referred to as Reference Example 1. After the first stretching, this was heated and baked at 370 ° C. for 5 minutes with the four sides of the porous film fixed.

The stretching conditions of Examples 1 to 5 and Reference Example 1 are shown in Table 1.
Shown in. In addition, the properties of the porous membranes of Examples 1 to 5 are shown in Table 2, and their SEM photographs are shown in FIGS. Example 1
The fiber diameters of the porous membranes manufactured in No. 5 are all 0.1
It was in the range of 0 μm to 0.2 μm. The fiber diameter of the reference example was 0.22 μm on average.

[0031]

[Table 1]

[0032]

[Table 2]

Examples 6 to 11 Thickness 0.2 m produced from PTFE fine powder ("Fine Powder CD123" manufactured by Asahi Glass Co., Ltd.) by a usual method of paste extrusion, roll rolling and lubricant drying.
An unsintered unstretched tape having a width of m and a width of 150 mm was first stretched about 5 times in the width direction (first stretching) using a biaxial stretching machine at a stretching temperature of 300 ° C. and a stretching speed of 50% / sec. Then, it was simultaneously stretched in the biaxial direction (second stretching). Stretching
All stretching was carried out at a temperature of 300 ° C. and 10% in Example 6.
/ Sec, 25% / sec in Example 7, 50% / sec in Example 8
Seconds, 75% / sec in Example 9 and 100% in Example 10.
Per second, and in Example 11, at six kinds of stretching speeds of 200% / second, they were simultaneously or sequentially stretched in a biaxial direction at a stretching ratio of 7.5 times. That is, the final stretched area ratio is 5 × of the original unsintered unstretched tape in any of the examples.
It was stretched 7.5 times 7.5 = 281 times. After stretching, the film was heated and baked at 380 ° C. for 10 minutes in a state where all four sides of the film were fixed.

Although the film was broken at the drawing speed of 200% / sec in Example 11, the film was not broken at other drawing speeds, and an extremely thin film having a thickness of 10 μm or less could be produced. The film thickness was significantly reduced as compared with the original unsintered unstretched tape. When the surface of the stretched film was observed by SEM, it was observed that the structure was composed of only fibers in which no knots having a large dimension of 1 μm were substantially observed. The film strength was significantly improved compared to the unfired unstretched tape.

Reference Example 2 The porous film after the first stretching used in the second stretching of Examples 6 to 11 is referred to as Reference Example 2. After the first drawing, this was heated and baked at 370 ° C. for 5 minutes with the four sides of the porous film fixed.

The stretching conditions of Examples 6 to 11 and Reference Example 2 are shown in Table 3. Table 4 shows the properties of the porous membranes of Examples 6 to 11.
Shown in. FIG. 8 shows an SEM photograph of the porous film produced in Example 8.

[0037]

[Table 3]

[0038]

[Table 4]

Comparative Example 1 The fiber structure of the porous membrane (FIG. 2) produced by the conventional method of stretching in an unfired state and the porous membrane of Example 5 (FIG. 7) were compared. The porous film produced by the conventional method is PTFE fine powder ("Polyflon.
Fine powder F104 ") with a thickness of 0.2 produced by the usual methods of paste extrusion, roll rolling, and lubricant drying.
mm, width 150 mm, unstretched unstretched tape is stretched by a biaxial stretching machine at a stretching temperature of 300 ° C., a stretching rate of 200% / sec in the width direction of the tape, and a stretching rate of 500% in the longitudinal direction of the tape. It was produced by stretching about 6 times per second. After the stretching, the porous film was heated and baked at 370 ° C. for 5 minutes with the four sides fixed.

[0040]

[Table 5]

[Brief description of drawings]

FIG. 1 SEM of a typical PTFE (uniaxial) stretched porous membrane
Photograph (magnification 3000x, same below).

FIG. 2 SEM of a typical PTFE (biaxial) stretched porous membrane
Photo.

FIG. 3 is an SEM photograph of the PTFE porous membrane produced in Example 1.

FIG. 4 is an SEM photograph of the PTFE porous membrane produced in Example 2.

5 is a SEM photograph of the PTFE porous membrane produced in Example 3. FIG.

FIG. 6 is an SEM photograph of the PTFE porous membrane produced in Example 4.

FIG. 7 is an SEM photograph of the PTFE porous membrane produced in Example 5.

FIG. 8 is an SEM photograph of the PTFE porous membrane produced in Example 8.

Claims (6)

[Claims] 1. A polytetrafluoroethylene porous film formed by stretching a non-sintered polytetrafluoroethylene in an unsintered state, the average size of which is 1 μm in diameter when observed by a scanning electron microscope. A polytetrafluoroethylene porous membrane comprising a fibrous structure having substantially no knots larger than a circle. 2. A polytetrafluoroethylene unsintered body is stretched at a stretching area ratio of at least 50 times in a uniaxial or more direction at a stretching rate of 100% / sec or less at a heating temperature of less than 327 ° C. A method for producing the polytetrafluoroethylene porous membrane according to claim 1. 3. A non-sintered polytetrafluoroethylene material is stretched at a stretching area ratio of at least 50 times in a uniaxial or more direction at a heating rate of less than 327 ° C. and a stretching rate of 50% / sec or less. A method for producing the polytetrafluoroethylene porous membrane according to claim 1. 4. A non-sintered polytetrafluoroethylene body is stretched at a stretching area ratio of at least 50 times in a uniaxial or more direction at a stretching rate of 20% / sec or less at a heating temperature of less than 327 ° C. A method for producing the polytetrafluoroethylene porous membrane according to claim 1. 5. The fired polytetrafluoroethylene porous film according to claim 1, which is obtained by heating the polytetrafluoroethylene porous film according to claim 1 to a temperature of 327 ° C. or higher. 6. The polytetrafluoroethylene according to claim 5, wherein the polytetrafluoroethylene porous film is obtained by the method according to any one of claims 2 to 4 and then heat-treated at a temperature of 327 ° C. or higher. A method for producing a fluoroethylene porous membrane.